Decision-Making on-board an Autonomous Agile Earth-Observing Satellite

In this paper, we describe the Earth observation mission we have been working on, including its requirements in terms of autonomy. Then, we describe the decision problem to manage, the reactive/deliberative architecture we used for on-line decision-making, the decision rules used by the reactive part, and the iterated stochastic greedy search used by the deliberative part. Finally, we present results of experiments performed on off-line and on-line realistic scenarios. The Earth observation mission In this paper, we present the main results of a study which has been performed in the context of the French CNESONERA-LAAS AGATA project (Autonomy Generic Architecture: Test and Applications (Charmeau and Bensana 2008)). The AGATA project uses several scenarios as examples of application of autonomy techniques. One of these scenarios, the one presented in this paper, is a speculative project of improvement of the agile Earth-observing Pleiades satellites which will be launched in 2010: a kind of post-Pleiades project. Agility means that these satellites are able to move freely around their inertial center along the three axes (roll, pitch, and yaw) thanks to gyroscopic actuators, while moving on their orbit. These satellites are each equipped with a high resolution optical observation instrument (see Figure 1) and observation of ground areas is performed thanks to the combined movement of the satellite on its orbit and on itself (see http://smsc.cnes.fr/PLEIADES/GP_gallery.htm Movies about Pleiades). The way of managing the Pleiades satellites that is currently considered is the following one: observation requests are sent by users to a centralized ground mission center; each day, this mission center builds a precise activity plan for each satellite, taking into account the current requests, their priorities, and the weather forecast; these plans cover all the satellite activities: orbital manoeuvres when they are necessary to maintain the satellite on its reference orbit, observation of ground areas, downloading of observation data to ground stations, pointing of the solar panels towards the Sun in order to recharge batteries, and attitude rendezvous to move from one activity to another one, for example from the end of the observation of a ground area to the beginning of the observation of another one; these plans specify the sequence of activities to be performed by the satellite with their precise temporal positions; they are executed by the satellites without any modification. However, because optical observation is sensible to the presence of clouds and because weather forecast on one day for the next one is imperfect, many observations (around 80% with the current optical Earth-observing satellites) are lost due to the presence of too many clouds.